Process for reducing diethylene glycol formation in poly(ethylene terephthalate)prepolymer

ABSTRACT

FORMATION OF CONTAMINANT DIETHYLENE GLYCOL BY-PRODUCT DURING MANUFACTURE OF POLY(ETHYLENE TEREPHTHALATE) PREPOLYMER BY ESTER EXCHANGE OF DIMETHYL TEREPHTHALATE AND ETHYLENE GLYCOL IN THE PRESENCE OF A CATALYST MIXTURE IS MINIMIZED BY INCLUDING LITHIUM ACETATE DIHYDRATE AS AN ESSENTIAL INGREDIENT IN THE CATALYST MIXTURE WITH ZINC ZCETATE DIHYDRATE AND/OR ANTIMONY TRIOXIDE.

United States Patent PROCESS FOR REDUCING DIETHYLENE GLYCOL FORMATION INPOLY(ETHYLENE TEREPH- THALATE) PREPOLYMER Kenneth T. Barkey, Rochester,N.Y., assignor to Eastman Kodak Company, Rochester, N.Y.

N0 Drawing. Original application Dec. 2, 1971, Ser. No. 204,365, nowPatent No. 3,749,697. Divided and this application Dec. 7, 1972, Ser.No. 313,047

Int. Cl. C08g 53/22, 39/04 U.S. Cl. 260--2.3 6 Claims ABSTRACT OF THEDISCLOSURE Formation of contaminant diethylene glycol by-product duringmanufacture of poly(ethylene terephthalate) prepolymer by ester exchangeof dimethyl terephthalate and ethylene glycol in the presence of acatalyst mixture is minimized by including lithium acetate dihydrate asan essential ingredient in the catalyst mixture with zinc acetate.dihydrate and/or antimony trioxide.

This is a division of application Ser. No. 204,365, filed Dec. 2, 1971,now Pat. No. 3,749,697.

FIELD OF THE INVENTION This invention relates to the preparation oflinear polyester of glycols and terephthalic acid. More particularly,this invention relates to an improved ester interchange process in whichglycol is exchanged with the methyl moiety in dimethyl terephthalate inthe presence of a cat alyst mixture whereby the amount of diethyleneglycol which forms as a contaminant by-product in the known prior artprocess is substantially reduced. The polyester product formed by thisimproved process has imparted to it physical properties which enhancethe subsequent proc essing of the polyester into a biaxially-orientedfilm form.

., ,DESCRIPTION OF THE PRIOR ART --.'The preparation of polyesters suchas poly(ethylene terephthalate) and poly(propyleneterephthalate) byester exchange: andcondensation is well known. U.S. Pats. 2,465,319,2,727,881 and 3,488,382 and many others describe such processes -ingreat detail. A common feature of a large number of these processes isthat they begin with a lower dialkyl ester of a bifunctionaldicarboxylic acid which is condensed with a bifunctional glycol, theglycol usually being used as such although it can be used in 'the formof lower-alkanoic acid ester thereof, such esters-being equivalents ofthe glycol. These processes involve'the initial preparation, in thepresence of a catalyst,-.of a substantially monomeric protopolymer underconditions facilitating removal of the lower alkanol formed from thelower dialkyl ester by ester interchange with the glycol. This issometimes referred to as the first stage or the ester exchange step ofthe polyester preparation and can be conducted under a variety ofconditions-using many different types of apparatus. For example,:thelower dialkyl ester and the glycol in a mole ratio beginning at about 1to 1.5 up to about 1 to 10 are placed in a reaction vessel equipped witha packed column with the vessel being heated at a temperature whichpermits the lower alkanol to pass through the column, with the glycolbeing retained by the column and returned to the reaction vessel byrefluxing, whereby the lower alkanol is theoretically removed duringthis first stage of the polyester. preparation which results in theformation of a monomeric protopolymer.

The catalysts which may be used in the preparation of polyestersaccording to the above processes include organic and inorganic compoundsof metals such as titanium, manganese, antimony, zinc, tin, lead,calcium, cobalt, lithium, combinations thereof, etc., many of which haveheretofore been utilized by those skilled in the art. The prior art, forexample, discloses a great number of such catalysts, some of which aredescribed in patents such as US. 2,465,319, US. 2,720,502, US.2,727,881, US. 3,488,382 and others. Specific catalysts heretofore knownand which may be utilized include tetraisopropyl titanate, titaniumdioxide, zinc acetate, zinc acetyl acetonate, lead oxide, calcium oxide,lithium ethoxide, antimony trioxide, manganese oxides and the like.Generally, the acetates, chlorides, nitrates, sulfates, oxides andalkoxides of one or more of the metals zinc, manganese, tin, lead,titanium, antimony, cobalt and lithium are preferred. For example, thecatalyst system of zinc acetate and tetraisopropyl titanate is wellsuited to attain the desired reaction activity. The catalyst (orcatalyst mixture) is generally utilized in a concentration of from about0.002 percent to about 0.2 percent by weight of the reactants beingcondensed. Higher or lower percentages can also be employed. Generallyfrom about 0.001 percent to about 0.05 percent catalyst can beadvantageously employed. Preferred ester exchange catalysts include, forexample, zinc acetate, manganous acetate, cobaltous acetate, lithiumacetate, and tetraisopropyl titanate. However, many others are known.

During the first stage manufacture of the above type of monomericprotopolymer polyester, specifically poly- (ethylene terephthalate)prepolymer, by means of catalytic systems using the above describedcatalysts, a byproduct compound, diethylene glycol (DEG) is formed andreacts with the polyester to form a copolyester which is randomlydistributed in the polyester, which itself is a homopolymer, from whichcopolyester it cannot readily be removed. The copolyester has a lowermelting point than does the parent homopolymer polyester and melts overa broader temperature range than does the polyester homopolymer.Furthermore, the copolyester nucleates and crystallizes differently thandoes the polyester homopolymer. As a result, during subsequentoperations involved in the manufacture of a film product from thepolyester product containing the copolyester as a contaminantby-product, many problems arise which have an adverse effect upon thequality and uniformity of the film product made therefrom and upon theprocessing operations necessarily used in making the film. For example,in the manufacture of the film product, the first stage polyesterproduct is first melted and then cast to form a readily grindable solidproduct for use in further polymerization steps. Uniform nucleation andrapid crystal growth are essential for casting of such polyesterprepolymer in an opaque readily disintegratable solid form. Presence ofthe copolyester contaminant in excessive and often varying amountscauses thecast polyester prepolymer to solidify in glassy, amorphousform which is diffiult to prebreak of crush, grind. and fluidize for thesubsequent further polymerization of the polyester prepolymer to ahigher degree of polymerization ina second polymerization stage in aknown manner. The adverse effectsof the copolyester contaminant are"carried ov'ei' into the melt from which the film product ultimately ismade when the completely polymerized polyester is ex-' truded. Adverseeffects such as increased incidents of tearoffs and breaks areencountered during tentering and drafting operations during whichthepolyester film is biaxially oriented. Prior to the present invention,the only known way to minimize the latter forms of defects was toincrease the inherent viscosity (I.V.) of the individual batches ofpolyester during the second stage ofpolym erization, thereby'causinglonger processing times and consequent loss of production capacity.

3.. SUMMARY OF THE INVENTION The present invention comprises the use inthe ester exchange step of polyester preparation of poly(ethyleneterephthalate) from dimethyl terephthalate and ethylene glycol of animproved catalyst mixture wherein lithium in the form of a salt of anorganic acid, e.g. lithium acetate dihydrate, is used in a bufferingamount in a two or three part catalyst system with catalytic amounts ofzinc, antimony or zinc-antimony catalyst. Preferably, the improvedcatalyst mixture consists essentially of lithium:zinc:antimony in theratio by parts per million (p.p.m.) of dimethyl terephthalate (DMT)charged to the reactor of about 20:65:300, preferably in the forms oflithium acetate dihydrate, zinc acetate dihydrate, and antimonytrioxide, respectively. In the latter forms, the amounts of theingredients used in the catalyst mixture, taken as percents of theweight of DMT charged, will be about 0.03% lithium acetate dihydrate,about 0.02% zinc acetate dihydrate and about 0.035% antimony trioxide,or about 0.12 parts lithium acetate dihydrate, 0.08 parts zinc acetatedihydrate, and about 0.14 parts of antimony trioxide per 388 parts ofdimethyl terephthalate charged to the reactor.

The polyester prepolymer formed as a result of the practice of thepresent invention is characterized in that the amount of diethyleneglycol present in the first stage polyester product is advantageouslylower than that present in corresponding prior art product, and themelting point of the first stage polyester prepolymer product issignificantly higher than that of the corresponding prior art product,using in each prior case a catalyst mixture consisting of zinc and/orantimony compounds. The polyester prepolymer product is characterized inthat it has advantageously improved casting, grinding and fluidizationproperties for the further manufacture of poly(ethylene terephthalate)powder, which powder subsequently is melt extruded into film form,drafted and tentered to form a biaxially-oriented poly(ethyleneterephthalate) film support in a known way for use in manufacture ofphotographic film products.

The following examples illustrate the advantages of a preferredembodiment of the invention over the prior art practice.

1 2 moles. i 4 moles. Based on weight of dimethyl terephthalate chargedto reactor.

TABLE II.PROCESS DATA AND PROPERTIES OF POLY- (ETHYLENE TE REPHTHALATE)PREPOLYMER Example 1 Example 2 Time to complete ester exchange reactio Vto form prepolymer, min 44 26 Inherent viscosity of prepolymer- 0. 44 0.43 Carboxyl radical content, meq./kg.- 36 14 Melting point (-O.) 256260-261 Diethylene glycol content, mole percent of 1 prepolymer 3. 5

Thus, it is evident from comparison of the Process Data and Propertiesof Poly(ethylene terephthalate) Prepolymers (Table II) made by the priorart method (Example 1) in the absence of lithium acetate dihydrate inthe catalyst mixture and the method of the present invention (Example 2)wherein lithium acetate dihydrate is present in the catalyst mixturethat the use of only a buifering amount of lithium acetate dihydrate(Table I, Example 2) has reduced the reaction time for the esterexchange reaction by 18 minutes, from 44 minutes to 26 minutes. Also,While the inherent viscosity of the pre- 4 l M Jr. I p polymer polyesterhas not been significantly affected, i.e. 0.44 and 0.43, the carboxylradical content, which under normal circumstances is indicative ofpresence of diethylene glycol, has been reduced greatly, from 36 to 14mole equivalent (meq.) per kilogram (kg.) of prepolymer polyester.Further, the melting point of the poly(ethylene terephthalate)prepolymer '(prepolymer polyester)'has' 5a: vantageously been raisedfrom 256 C.'to 260261C. by practice of the present invention. And,finally, the actual amount of diethylene glycol present in.said'prepolymer polyester has been reduced by two full percentagepoints, i.e., from 3.5 mole percent to 1.5 mole percentin the prepolymerpolyester. In practice of the invention, it has been found that loweringof the mole percent of diethylene glycol present by one mole percentresfults -in an increase in melting point of the polyester prepolymer by2.15 C. In the present example, a reduction of 2 mole percent indiethylene glycol content of the polyester prepolymer has resulted in a4 to'5 C. increase in melting point. The prepolymer polyester thus hasphysical propf erty characteristics which one skilled in the art of polycondensing the polyester prepolymer to the polyester and the manufactureof the so-obtained poly(ethylene terephthalate) into film form by knownprior art methods will readily recognize as advantageous for the formingof bi=' axially-oriented film therefrom.

Procedures for carrying out the polycondensation process and proceduresfor forming biaxially-oriented film from poly(ethylene terephthalate)are described, for example, in US. 3,043,564 and US. 3,048,564, and suchprocedures may be used to make the polyesterandfto makebiaxially-oriented film from the poly(ethylene terephthalate) of thepresent invention. When the poly(eth.-,.

ylene terephthalate) of the present invention is being tentered anddrawn for the making into film, the adverse effects discussed above,e.g. tearoifs and breaks, are found to be absent or considerably reducedin frequency While the amount of lithium used in practice. of theinvention has been shown in Example 2 to be 21- p.p.m.;

of lithium as Li (equivalent to 0.1176 g. of lithium ace:

tate dihydrate) in a reaction mixture of two gram moles of dimethylterephthalate with four gram moles of ethylene glycol, resulting in theformation of two gram moles of poly(ethylene terephthalate) prepolymer,.the advantageous results of the invention also may be obtained by theuse of from about 15 to about p.p.m. of lithium as Li (equivalent toabout 0.078 g. to about 0.340 g. of lithium acetate dihydrate) in areaction mixture of two gram moles (388 g.) of dimethyl terephthalatewith four gram moles (248 g.) of ethylene glycol. The amounts ofantimony catalyst then used may range from about 100 p.p.m. to about 500p.p.m. (equivalent to from about 0.046 g. to about 0.230 g. of antimonytrioxide) in the same reaction mixture. The amounts of zinccatalyst-then used may range from about 32 p.p.m. to about 130 p.p.m.(equivalent to from about 0.042 g. to about 0.168 g. of zinc acetatedihydrate) in the same reaction mixture.

Preferably, the improved catalyst mixture of this iIlVCIh I tion willconsist of p.p.m. of zinc, 300p.p.m.':of

I terephthalate.

antimony and from 21 to -35 p.p.m." of lithium in the form of theequivalent amounts of zinc acetate dihydrate, antimony trioxide andlithium acetate dihydrate, respec: tively, based on the weight ofdimethyl terephthalate charged to the reactor. 7 "p,

EXAMPLE 3 TABLE III P.p.m. catalyst Prepolymer, inherent Melting Batchnumber Zn 1 ,Sb 1 Li a viscosity point C.)

1 Charged as zinc acetate dihydrate.

! Charged as antimony trioxide.

8 Charged as lithium acetate dihydrate.

From the data it is seen that the use of Li (lithium acetate dihydrate)gave poly(ethylene terephthalate) prepolymer melting 2 C. higher in eachbatch (1b and 2b) where it was present as compared to those batcheswhere lithium was absent (1a and 2a), and is indicative of the fact thatless diethylene glycol was formed even with water present in the glycol.

.The advantageous effects of the presence of lithium in the catalystmixture for reducing the content of contaminant diethylene glycol in theprepolymer polyester have also been found to be obtained when lithium isused in combination with zinc as the only other catalytic material inthe catalyst system-or in combination with antimony as the .only othercatalytic material in the catalyst system. Examples 4 and 5 furtherillustrate such advantageous results obtained -by practice of theinvention. In the examples it is to be noted again that the obtainmentof a higher melting point for the prepolymer polyester is' indicative'of the lowering of the diethylene glycol content in the polyester. 7

EXAMPLE 4 Distilled monomer (250' g.) (i.e. poly(ethylene terepthalate)prepolymer) separated from poly(ethylene terephthalate) prepolymer in arecovery still and containing 280 p.p.m. of zinc and 150 p.p.m. ofantimony catalysts (based on weight of monomer charged) was reactedadditionally with new dimethyl terephthalate (388 TABLE IV" i Y IProperties of prepolymer v Y P.p.m. added catalysts Batch COOH. M.P.

Inherent viscosity.

'From the data, it is -to be .noted that inclusionof lithium in thecatalyst mixture again caused lesser amounts of diethylene glycol toform (higher melting point) and lesser polyester acidity as evidenced bylower carboxyl content (15 meq./-kg. of COOH versus 29 meq./ kg. ofCOOH) for the poly(ethylene terephthalate) pre polymer.

EXAMPLE 5 The recovery of waste poly(ethylene terephthalate) from filmmanufacturing operations is an ecological and economical necessity.Practice of the invention in such recovery is shown by the followingdata.

Scrap poly(ethylene terephthalate) from film manufacturing operationswas subjected to glycolysis reactions to obtain polyester of lowermolecular weight for reprocessing to film grade poly(ethyleneterephthalate). Various catalyst systems were used to accelerate thebreakdown of the polyester and to minimize diethylene glycol formation.The scrap poly(ethylene terephthalate) had the following characteristicsbefore glycolysis:

TABLE V Inherent viscosity (I.V.) 0.65 Carboxyl (COOH), meq./kg. 35Melting Point, C 257 In each of the following batches, A-D, 300 g. ofpoly (ethylene terephthalate) were reacted with 450 g. of

ethylene glycol (1.5 weight ratio of ethylene glycol to scrappoly(ethylene terephthalate)) at the boiling point (l98205 C.). Timesnecessary to give clear solutions varied depending upon the catalystsystem and its concentration. After the scrap had been dissolved andreacted to low molecular Weight oligomers, excess ethylene glycol wasdistilled off and the melt was polymerized for two hours at 260 C. andat a pressure of less than 100 microns (0.1 mm. Hg). The resultingpoly(ethylene terephthalate) prepolymer was cast and analyzed. Theresuling data are shown in Table VI.

1 Charged as zinc acetate dihydrate. 9 Charged as lithium acetatedihydrate.

g.) and additional new ethylene glycol. In batch a, 300 p.p.m. of Sbwere added (as Sb O in batch b, 21 p.p.m. of Li (charged as lithiumacetate dihydrate) and 300 p.p.m. of Sb (as Sb O were added. The resultsTo eliminate the effect of low molecular weight, as evidenced by theinherent viscosity (I.V.) of the prepolymer and its melting point,batches B and D were polymerized further by solid phase buildup at 210C. for seven obtained are shown in Table IV. hours. The results obtainedare shown in Table VII.

TABLE VII P.p.m. catalyst 0 hour 1 hour 2 hours 5 hours 7 hours 7 BatchCOOH COOH COOH COOH COOH M.P number Zn Li 1 I.V. (meq. g.) I.V.(meq.lkg.) I.V. (mam/kg.) I.V. (meq./kg.) I.V. (meq.lkg.) C.)

1 Charged as zinc acetate dihydrate. i Charged as lithium acetatedihydrate.

polymerization for..7-hours;in each. batch,..batch -D,. which had 52,p.p.m, of lithiumpresent along with 162 p.p.m. of zincinthecatalyst-mixture;resulted; in arecycled poly- '(ethyleneterephthalate) product'havingQOOI-I content sof 38 meq./kg., comparedto-original-COOH-contentof meg/kg. in the scrap polyester and ajmeltingpoint-of 253 compared-tothe original of- 257 0. The results from batch Bshow that in the absence of lithium, the zinc cata- -lyst alone'gavepolyester product which had 53 meq./kg. of COOH(comp'ared to 38"for D')and'a melting point ofonly 247 (-7.5 mole percent DEG) compared to 253'C. '(5 mole percent DEG). Therefore, it is evident that use of lithiumacetate dihydrate in the catalyst mixture is advantageous for the scrappolyester recovery process as well as for the manufacture of freshpoly(ethylene terephthalate) prepolymer. The recycled prepolymer is in aform suitable for polycondensation to a poly(ethylene terephthalate)which can be made into a useful film product.

As used throughout this description, it is to be understood that theinherent viscosity (I.V.) of poly(ethylene terephthalate) prepolymer isin the range of about 0.40 to 0.44 and the inherent viscosity ofpolycondensed polyester, .i.e. poly(ethylene terephthalate) is in therange of about Although the nature of the activity of the lithiumacetate dihydrate in inhibiting the formation of diethylene glycol inaccordance with the invention is not definitely known, it is believedthat the acidity of the reaction mass is in some manner affected.However, when experiments similar to those given in the examples werecarried out using sodium acetate as a substitute for the lithium acetatein the catalyst mixture of the invention the results obtained were notas good and, in addition, the biaxially-oriented films made from thesodium acetate experiments were found to be hazy and not clear as werethose obtained when lithium acetate dihydrate was used in the catalystmixtures of the invention.

The invention has been described in detail with particular reference topreferred embodiments thereof, but it will be understood that variationsand modifications can be effected within the spirit and scope of theinvention.

Lclaimg I y In a prqcess for,;recovering .waste poly(ethyl eneterephthalate). from filmmakin operations by glycolysis of said waste wih ethylene glycol'at arr elevatecltemperat-ure-in the presence otf acatalyst to formpoly (ethy-lene terephthalate) innrthe form ,of; low..molecular, weight oligomersathe improvement which comprises .carrying,out

said glycolysis inythe presence: of-a catalyst mixturecOn- -sisting;essentially of-.(a) 1a diethylene glycol-formation inhibiting amount oflithium acetate dihydratexwith (b-) a catalytic amount of at least oneof zinc acetate dihydrate or antimony trioxide;

2. The process of claimlwherein said catalyst mixture contains (a) about1 5 to aboutl p.p.m. oi'lithi'rrhtgi'fb) O to about 500 p.p.m. ofantimony, 'and (c) 0 to about 130 p.p.m. of zinc, based on' the weightof'waste poly- (ethylene terephthalate) charged tothe reactor.

3. The process of claim 2 wherein said catalystjmixture contains (a)about 21 to 35 p.p.m. of lithium b) 'abput p.p.m. of zinc and (c)' about300"p.p.'m.'of'antimony.

4. The process of claim 2 wherein said catalyst mixture contains (a)about 21 p.p.m. of lithium, [(b) about '65 p.p.m. of zinc and (c) about'30'0 p.p.m. of antimony.

5. The process of claim 2 wherein the catalyst mixture contains about 52p.p.m. of lithium andabout 162 p.p.m. of zinc. .v

6. The process of claim 2 wherein said low molecular weight oligomersare further polymerized by solid phase buildup at polymerizingtemperatureto increase-their inherent viscosity to a viscosity in therange of about-0.60 to about 0.65. s s

References Cited UNITED STATES PATENTS 252-426, 430; 260- R, 475 D, 475P

